1. Field of the Invention
The present invention relates to a fiber laser system, and more particularly, to a light coupler including an optical fiber bundle based on either a single-mode core fiber or a few-mode core fiber and a fiber laser system including the light coupler.
2. Description of the Related Art
Conventional fiber lasers may be divided into two types: single cladding optical fiber lasers and double cladding optical fiber lasers. A single cladding optical fiber laser is a laser using an optical fiber including one core and one cladding as a gain medium. The core of the single cladding optical fiber laser is generally formed of a material enabling light pumping; e.g., silica-based glass to which rare-earth elements are doped. Furthermore, since light only travels through the core, pump light should be incident into the core, and laser is emitted from the core. Since the core of the single cladding optical fiber laser has a relatively small cross-section, the amount of pump light incident thereto is limited, and thus the output power of the laser emitted from the core is limited. Meanwhile, the cross-section area of a cladding layer is tens to hundreds times larger than that of the core. Thus, if pump light can be incident into the cladding, the amount of pump light incident thereto may be significantly increased.
A double cladding optical fiber laser is an optical fiber laser in which pump light can be incident into a cladding layer. Generally, the double cladding optical fiber laser includes a single-mode core and a plurality of cladding layers. The plurality of cladding layers includes an inner cladding layer and an outer cladding layer, both of which are displaced outside the core. The inner cladding layer surrounding the core has a cross-section larger than that of the core and has a high numerical aperture. The outer cladding layer is displaced outside the inner cladding layer, and may be generally formed of a polymer having low refraction ratio. The refraction ratio of the core is slightly greater than that of the inner cladding layer, and the refraction ratio of the inner cladding layer is larger than that of the outer cladding layer. Due to such differences of refraction ratios, pump light incident through the inner cladding layer is totally reflected at the interface between the inner cladding layer and the outer cladding layer, and thus the pump light travels along the inner cladding layer and is not emitted to outside. Thus, pump light having low light quality may be easily incident into the inner cladding, and the incident pump light travels along the inner cladding and is absorbed by rare-earth elements in the core. High quality and high power laser is emitted by the rare-earth elements that are excited by the pump light.
Examples of the most popular high power pumping light source include a semiconductor array type laser diode, and various methods can be used to make a pump light incident into a double cladding optical fiber. In one of the methods, light generated by a semiconductor array type laser is made incident into an inner cladding of a double cladding optical fiber by using bulk optics. According to this method, a single-mode optical fiber laser over 1 kW can be emitted. The method is disclosed in “Optics Express (vol. 12 (25), pp. 6088, 2004).” In addition, U.S. Pat. No. 5,999,673 discloses a beam launching technique which couples a light generated by a semiconductor array type laser through an outer side of double clad fiber. Recently, a laser system for emitting laser of higher quality and power than the laser systems described above is required.